I'm just gonna throw some more science into this discussion if no one minds. (Sorry for long post.)
First off, bongs deliver THC through smoke, produced by combustion. Now, the problem with combustion is threefold:
1. At temps over 800C THC and other cannabinoids start to break down. In conclusion the potency drops, since THC and cannabinoid quanity drops.
2. Combustion transforms aromatic esters into tar. Tar is extremly toxic to lungs. Still, a recent study of the Maryland Collage found out that heavy cannabis smoking will not produce precancerous formations in lungs, tar being eliminated completly from the lungs. This still doesn't mean that tar will not affect the respiratory passages, trachea, brochi, etc.
3. All remaining cellulose will be broken down into carbon monoxide (CO). CO is highly toxic. It binds with red cells, hindering any further oxigen transportation by the affected red cells and will inhibit motility of most white cells, lowering immunity. The effects of CO will dissapear completly only after 120 days after a person is no longer exposed to CO.
Bongs have only 2 roles: cooling smoke and filtering it. Sadly, filtering the smoke through water will only remove ash, which is not the worst enemy of our lungs. Tar and CO manage to pass through water effortlessly. Don't get me wrong, I also enjoy bongs, it's just that you should know what you're inhaling.
The truth is bongs do deliver less THC. What is not mentioned is that this is in comparison to vaporizers or vaporizing. That is why everyone is so amazed at this conclusion, since they believe it is in comparison to joints. Joints deliver even less THC. The reason for this is that joints, as well as bongs use combustion, at temps high enough to break down THC and cannabinoids.
Also, between inhales, the buds cool down to some 400-500C. This temp is low enough to permit a swift "evaporation" of cannabinoids. Practically, the smoke that's coming off your bowl or joint tip while you're not inhaling has more THC then the one you inhaled, because when air is pulled towards the flame/burinig plant matter, it heats up, having more oxigen to sustain itself.
Vaporizers, on the other hand provide the following advantages: no tar or CO is produced, no THC or cannabinoids are broken down, completly all cannabinoids are inhaled. Vaporizing means heating up at some 250-350C, below the point of combustion. Since most vaporizers do not permit any vapor to escape, all cannabinoids that evaporate can be inhaled. From my personal experience, vaporizers are far more efficient than bongs or joints.
And now to offer a solution that will make everyone happy. If you love bongs, like I do, but you feel that you care about your health and don't want to waste one drop of resin, buy a VaporStar.
I have a 50cm no name glass ice bong with diffuser and I use it with a VaporStar. It works perfectly, I don't loose any vapor, and can get high like a kite. If you like joints or pipes, I recommend an Ubie or VaporGenie or even an electrnic vaporizer. They all work great, but since I'm a bong lover, I enjoy the VaporStar most of all.
Hope that cleared some issues. I'd like to thank BCtrippin and Landragon for being serious and factual about this subject.